ATtiny261A datasheet(123/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

13. USI – Universal Serial Interface

13.1 Features

• Two-wire Synchronous Data Transfer (Master or Slave)

• Three-wire Synchronous Data Transfer (Master or Slave)

• Data Received Interrupt

• Wakeup from Idle Mode

• In Two-wire Mode: Wake-up from All Sleep Modes, Including Power-down Mode

• Two-wire Start Condition Detector with Interrupt Capability

13.2 Overview

The Universal Serial Interface, or USI, provides the basic hardware resources needed for serial

communication. Combined with a minimum of control software, the USI allows significantly

higher transfer rates and uses less code space than solutions based on software only. Interrupts

are included to minimize the processor load.

A simplified block diagram of the USI is shown in Figure 13-1 For actual placement of I/O pins

refer to “Pinout ATtiny261A/461A/861A” on page 2. Device-specific I/O Register and bit loca-

tions are listed in the “Register Descriptions” on page 130.

Figure 13-1. Universal Serial Interface, Block Diagram

The 8-bit USI Data Register (USIDR) is directly accessible via the data bus and contains the

incoming and outgoing data. The register has no buffering so the data must be read as quickly

as possible to ensure that no data is lost. The data register is a serial shift register where the

most significant bit is connected to one of two output pins depending of the wire mode configura-

tion. A transparent latch between the output of the data register and the output pin delays the

change of data output to the opposite clock edge of the data input sampling. The serial input is

always sampled from the Data Input (DI) pin, regardless of the configuration.

Two-wire Clock

Control Unit

(Output only)

DI/SDA

(Input/Open Drain)

USCK/SCL

(Input/Open Drain)

4-bit Counter

USIDR

USISR

USICR

CLOCK

HOLD

TIM0 COMP

[1]

USIDB

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